How does H-S Precision deliver awesome accuracy in a 6 lb rifle?

Barrel weight has little, if anything, to do with accuracy. Each one whips at its own resonant frequency and harmonics thereof for every shot fired. And that doesn't change with the ammo used. The amount will be a bit more with heavier loads but the frequencies stay the same.

Much more important is the barrel's groove diameter relative to the bullet's diameter. Bullets need to be a few ten-thousandths bigger than the barrels groove diameter for best accuracy.

The above assumes the barreled action is properly fit to the stock so it stays in the same place for each shot. And that the chambers are well centered on the bore axis. Plus the barrel's bore/groove diameters are the same all the way from leade to muzzle.